In the modern technological world many systems require heat to be either added or dissipated towards maintaining their operability and enhancing their thermodynamic efficiency. This change in gas/liquid temperature is typically provided by a heat exchanger, which generally operates by thermally connecting two streams which have a thermal potential difference. Due to form factor limitations associated with many size restrained applications, the state of the art is advancing towards more compact designs. This forms the basis towards higher performance and efficiency heat exchangers—enabling more heat transfer for the same size heat exchanger unit. The term “thermal efficiency” is used in this disclosure to describe the net heat exchange per unit volume of the heat exchanger, while the “performance” is used in this disclosure to describe the net heat exchange achieved per pressure loss in the fluid flow through the heat exchanger unit.
The net total heat exchange can be augmented by increasing the surface area of contact with the cooling surface, but the downside of this is the increased volume of the heat exchanger. In order to enhance the thermal efficiency of conventional designs, in Chinese patent No. CN 102829652 for “High-Efficiency Heat Exchanger based on Infrasonic Wave” to Zhejiang University of Science and Technology, infrasonic waves are proposed for periodic excitation of the tube heat exchanger. In that publication, there is described a tube heat exchanger based on infrasonic waves operating in the region between 4 and 14 Hz. The high-efficiency heat exchanger adopts a boundary layer control mode by infrasonic waves; thereby enhancing the thermal efficiency of the conventional tube heat exchanger. No further details are given in CN 102829652 regarding the physical mechanism behind the operation of this device, but it is believed that traveling waves at the appropriate infrasonic frequency create a viscous layer inside the conventional boundary layer of an attached flow heat exchanger. This then forms the basis of a second order boundary layer phenomenon known as steady streaming, where small amplitude periodic free stream oscillations induce a steady velocity component in the near wall region due to the non-linear boundary layer response. However, for this phenomenon to be relevant, the flow should be attached to the walls of the heat exchanger, as is typical for shell-tube heat exchangers of the type described in CN 102829652, and should not include any boundary layer separation and consecutive reattachment, as encountered in heat exchangers equipped with perturbators.
In the quest for attaining higher thermal efficiencies, compact heat exchangers (CHEs) have been developed, which attempt to provide high heat exchange rates in confined volumes. This can be achieved by means of designs having a large heat transfer surface area per unit of volume, which result in a higher thermal efficiency than more conventional designs such as shell-and-tube. Common CHEs designs include, but are not limited to:                Plate heat exchangers.        Plate-fin heat exchangers.        Printed circuit heat exchangers.        Spiral heat exchangers.        
Typically, in contrast to the smooth walls of shell-tube type heat exchangers, the heat exchange surfaces of CHEs are lined with perturbators or turbulators, which have two primary effects—firstly they increase the heat exchange surface “wetted” by the fluid, and secondly—they promote turbulence by locally separating and reattaching the fluid flow to enhance heat transfer to the surface. The latter is generally the dominant process, and such flow turbulence may include the previously mentioned boundary layer separation and consecutive reattachment, which may be considered important features for the improved heat transfer characteristics of CHE's. Reference is now made to FIG. 1A to FIG. 1I, which illustrate several different configurations of perturbators which are used in the flow path over the plate elements of a compact heat exchanger. The drawings show plan views looking down on the flow, and show respectively from FIG. 1A to FIG. 1I, protrusion patterns which are known in the art as 1A—90° continuous rib, 1B—60° parallel broken rib, 1C—60° V-shaped broken rib, 1D—an alternative 60° V-shaped broken rib, 1E—60° parallel continuous rib, 1F—60° V-shaped continuous rib, 1G—conventional zigzag, 1H—S shaped ribs, and 1I—Airfoil shaped ribs. In addition to rib type perturbation elements, pins, fins or dimples can also be used on the plates. The overall thermal performance depends upon the employed perturbation technology, on the geometric configuration of the flow passage including the profiles, height, pitch and angle of any perturbations, on the fluid flow rate, and on the flow channel aspect ratio.
However, there is a design compromise, which constrains the selection of the perturbation technology deployed in the heat exchanger. As a general trend, perturbators more effective in promoting heat exchange tend to further obstruct the fluid flow through the heat exchanger passageways. As a result, a pressure penalty is imposed on the heat exchanger; the more obstructive the employed perturbators, the higher the power of the pump or fan or compressor required to drive the flow past, around or through the perturbations. Therefore, a compromise has to be made between the desired heat exchange and the allowable pressure drop.
Reference is now made to FIG. 2, which is a presentation graph illustrating the performance of a large number of prior art perturbator technologies, as typically employed in commercial compact heat exchangers. The ordinate of the graph shows the Nusselt number Nu of the flow passages, normalized to the Nusselt number Nu0 of a flat plate, namely Nu/Nu0. The abscissa of the graph shows the friction coefficient f of the flow passages, normalized to the friction coefficient f0 of a flat plate, namely f/f0. The points on the graph represent the actual performance of different types of heat exchangers, having different configurations of perturbation elements, if used. The symbols shown correspond to the following configurations of heat exchange surfaces:
x—rib turbulators
Clear triangle—dimple protrusion plates
Filled triangle—pin finned plates
o—dimple-dimple plates
Filled square—swirl chambers
+ sign—plates with roughened surface
●—Dimple-smooth plates
Clear square—Smooth channel
The aim of heat exchanger technology is to provide as high a Nusselt number as possible, in order to improve the heat transfer efficiency, and as low a friction coefficient as possible, in order to reduce the pressure drop across the heat exchange path and improve thermal performance. This is shown by the arrows on the axes defining higher thermal exchange efficiency as the Nusselt number rises, and reduced performance in terms of the pressure drop across the heat exchanger path, as the friction coefficient rises. For a prescribed friction coefficient (fixed location on the abscissa), the perturbator configurations corresponding to increased heat transfer enhancement (higher ordinate) relate to superior heat exchange technologies with greater thermal efficiency. However, as a general rule of thumb, increased thermal efficiency comes at the cost of reduced performance as is shown by the gradually rising band of parameters demonstrated in FIG. 2.
In the design process, the heat exchanger configuration in terms of FIG. 2 is selected by determining the heat transfer enhancement required as a function of the acceptable pressure drop generated down the heat exchanger flow path. The performance requirements of the heat exchanger mandates the pressure requirements of the flow mechanism through the heat exchanger, and for any given configuration, a predetermined heat exchange requirement may require provision of the appropriate pressure-generating device.
Therefore, it would be desirable to provide a general upward shift of the performance/efficiency band in order to enhance the operation of the heat exchanger. This can be achieved by a technology providing augmented thermal efficiency without increasing the passage friction accordingly, such that improved thermal efficiency is obtained without the need to increase the input fluid pressure.
The main use of heat exchangers in industry is for extracting heat from surfaces by the relatively cooler fluid flow, and this disclosure has been prepared in terms of such a configuration. However, it should be understood that heat exchangers are also used for their heating function, and this disclosure is not intended to be limited to either one or the other heat transfer functions.
There therefore exists a need for a compact heat exchanger which overcomes at least some of the disadvantages of prior art systems and methods.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.